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Poor Compression After Top End Rebuild

This time last year I checked the compression on my bike and found it to be 125 psi L/ 130 psi R. Those numbers combined with a chipped exhaust valve inspired me to redo the top end. Heads and cylinders went off to a BMW dealer of note, well experienced with airheads and performing in house work. Cylinders were checked, heads had the seats milled and new valves installed. Exhaust valves had new guides installed as well. I resinstalled the heads with new rings, careful to orient the gaps per Oak's manual. Fired the bike up for the first time in the spring with no oil in the cylinders and held rpm at ~ 3000 for a minute. Then went for a 60 mile ride and dumped the oil. Ran a second course of dino 20W50 with some break in additive (ZDDP stuff mainly) and after 250 miles changed that to my preferred 20W50 semi syn (Brad Penn).

Bike ran fine, seemed a bit peppier than last season, so I thought. Didn't campaign at all this year, other issues both bike and nonbike kept me close to home. With only 1200 miles on the season, changed the oil out this week and tucked the bike into storage (snow has arrived in the Great White North).

Here's the kicker....did a compression test after my last ride (carbs off) and both sides are exactly the same at 120 psi. WTF???

Did I not run dino oil long enough? Have I not given it enough time? How long should it take to seat the rings??

Anything else I could be pondering?

Bike is tucked away till spring, won't be investigating until April now....

Last edited by wecm31; 11-24-2012 at 04:02 PM.
Reason: spelling

Gerald P
The last thing I want to be is just like everyone else...
1985 R80RT
1969 Sport Fury Convertible

Sounds like you did everything right...that first minute should have done quite a bit to seat the rings. Lots of varying speed, accels, decels for that first 60 miles. How have the valves been during this time? Does the bike burn any oil that you can tell? If rings are installed upside down, they can literally pump oil into the combustion chamber.

You might have to do a leakdown test in order to find out where the compression is leaking off to. Might be the rings are still seating...I would probably have run the dino oil longer.

I assume you had clean ring grooves. Ring break in has two distinct components. One is the ring to cylinder wall seal. Your scuff in probably did that OK. But the other part of ring sealing is good sealing of the rings to the lands adjacent to the grooves. Combustion pressure is supposed to keep the rings tight against the land, and combustion gasses behind the rings also exert an outward pressure.

Good ring sealing in the grooves requires a slight build up of varnish in the groove. I'd be pretty sure that dino oil is much better at this than synthetic.

Paul Glaves - "Big Bend", Texas U.S.A
"The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russellhttp://web.bigbend.net/~glaves/

I had the pistons cleaned by the dealer, they were ultrasonic soaked I think, came back to me looking like new. I was very careful to follow Oak's manual on the rings, quite confident I put them on right.

First ride was lots of on/off throttle, valve adjustment remained constant.

Brad Penn makes a 30 weight break-in oil. I typically use it for 100-200 miles on VWs and used it on my Airhead (new 1st OS bore) last summer. No ring issues that I can tell, but I've not run a compression test either.

I'd be pretty sure that dino oil is much better at this than synthetic.

I know this will sound blasphemous, but the former Butler & Smith mechanic that worked on my '73 R75/5 last year used no oil whatsoever when he reassembled the top end, other than the valves.

He explained that the B&S training philosophy (provided by BMW at the time) was that oil used during assembly basically prevents the rings from seating and tends to glaze the cylinder walls within several hundred stokes. He also claimed that break-in time would be reduced.

Yeah, right, I believe that.

But it worked.

"It is what you discover, after you know it all, that counts." _ John WoodenLew Morris
1973 R75/5 - original owner
1963 Dnepr